The "effect" this definition refers to is the theme, the message the author is trying to get across to the reader: sometimes called a "life-truth." A theme is the main idea the author wants the reader to come away with after finishing his/her story. Theme is defined as...

...the central and dominating idea in a literary work.

"A White Heron" is about a child named Sylvia, chosen by her grandmother (Mrs. Tilley) from among her siblings living in a "crowded manufacturing town," to live with her in the country. Sylvia loves it:

...this was a beautiful place to live in, and she never should wish to go home.

The theme is about the nature—that it is to be revered and protected.

One day as Sarah is walking the cow home, she hears a whistle:

Suddenly this little woods-girl is horror-stricken to hear a clear whistle not very far away. Not a bird's whistle, which would have a sort of friendliness, but a boy's whistle, determined, and somewhat aggressive. Sylvia left the cow to whatever sad fate might await her, and stepped discreetly aside into the bushes, but she was just too late. The enemy had discovered her...

Jewett's choice of the word "enemy" to describe the stranger in the wood sets the tone for Sylvia's interaction with the young man. He is affable enough, but his quest will help the innocent Sylvia better understand her own priorities, even at the age of nine.

It is interesting to note that "Sylvy" sees the young man as a danger, though she does not at first understand the ramifications of his intentions. The young man wants to find the white heron, kill it and preserve it for his collection—he is an ornithologist. He asks if he might stay the night and leave early to hunt. Mrs. Tilley welcomes the stranger with promise of a meal and a place to rest. As they talk, Mrs. Tilley notes that Sylvia is one with nature here...

There ain't a foot o' ground she don't know her way over, and the wide creatur's counts her one o' themselves...

This is of great interest to the young man who suddenly realizes that Sylvia could help him find the heron, and he offers a ten-dollar reward. Sylvia is an innocent, but knows how hard money is to come by. The next morning, she goes out early to find the bird for the man, but when she sees the it, her mind is changed.

...the heron has perched one a pine bough not far beyond yours...

She knows his secret now...But Sylvia does not speak...she must keep silence!...she cannot tell the heron's secret and give its life away.

It seems only right that these two characters with names that refer to the woods, would find their home there. However, for Sylvia, she also finds her heart there, and will not be forced to sell what she loves in the face of a reward or her grandmother's rebukes.